This research program is directed towards the study of erythrocytes in growing newborn animals. Emphasis will be placed in the investigation of the transition from fetal to adult characteristics of membrane transport properties of red blood cells in newborns of various mammalian species which have red cells of high and low potassium concentration, or high and low glucose permeability. Red cells of varying ages will be separated by means of an elutriation centrifugation technique for the study of changes in membrane transport function. Particular attention will be focused on the characterization of the kinetics of transport of K and Na and metabolically important sugars in fetal and adult red cells and also in maturing red cells in vivo and in vitro. In addition attempts will be made to measure single cell K, Na, and Fe in bone marrow cells separated by elutriation centrifugation technique. It is our hope that information gained in this study will contribute not only to the understanding of the maturation of membrane function for transport processes but also to the understanding of hemolytic diseases which are primarily due to the alteration of membrane transport function during erythropoiesis.